Demountable tape-automated bonding method

ABSTRACT

A Demountable Tape-Automated Bonding System for providing connections to a chip is disclosed. The chip is attached to or is held in place on a TAB frame that includes a generally flexible dielectric film which bears a pattern of conductive traces. A multitude of closely-spaced contacts which protrude downward from the chip contact the conductive traces on the TAB frame. The chip may be maintained in its proper location on the TAB frame by either a bonding agent or by compressive forces supplied by a cap which is fastened to the TAB frame and to a substrate, such as a printed circuit board, below the TAB frame. The substrate carries an array of conductive traces around the edges of the substrate. These traces match the traces on the TAB frame. The conductor traces on the TAB frame and on the substrate are held in contact with each other by compressive forces supplied by the cap which is fastened to the TAB frame and to the substrate. An optional bottom plate may be affixed beneath the substrate to provide additional strength and rigidity to the assembly. An optional heat sink may be attached above the cap to conduct away heat generated by the chip. An alternative embodiment of the invention includes an elastomer which is inserted between the cap and the substrate to cushion the compressive forces borne by the chip. The invention may also utilize a heat spreader placed directly above the chip. The Demountable Tape-Automated Bonding System constitutes a major improvement in the electronics industry which provides reliable, removable connections to delicate integrated circuit chips, eliminates the need for soldering, greatly diminishes alignment and bonding requirements, and offers a modular chip package that guards against mechanical, chemical, and electrical threats to its contents.

CROSS REFERENCED TO RELATED APPLICATION

This is a divisional of application Ser. No. 07/736,440, filed on Jul.26, 1991. Now U.S. Pat. No. 5,142,444 issued Aug. 25, 1992

BACKGROUND OF THE INVENTION

The present invention includes methods and apparatus for makingconnections to an integrated circuit chip. The DemountableTape-Automated Bonding System provides a modular, reliable, andself-aligned device that overcomes the main drawbacks of conventionalchip interface technology.

Each year integrated circuits become more powerful and capable ofstoring more information. One of greatest challenges confrontingdesigners in the electronics industry is finding more efficient andreliable methods to access the complex circuitry that resides within thechip package. A large rectangular IC chip having sides less than onehalf inch in length may have as many as five hundred or more leadsextending from it. Each of these conductors must be physically bonded orcoupled to external devices. These conductors are spaced together soclosely that the center-to-center distance, or "pitch", across a pair ofleads may measure less than five mils. As the leads become finer andfiner, and as they are arranged in more tightly packed arrays, chancesfor mechanical failure and electrical short circuits quickly rise tounacceptably high levels.

The basic methods of mass-producing connections to memory or logic chipsutilize a technique called "tape-automated bonding". This fabricationprocedure is commonly referred to by the acronym "TAB" and is well knownto persons ordinarily skilled in the electronics packaging art. Acontinuous insulative tape which is similar to photographic filmprovides a planar support for chips that are attached to individualsections or frames of the tape. TAB frames are generally rectangular orsquare portions that are arranged side-by-side along an uncut tape. Aspider-like metal pattern of conductive traces is formed on each frame.The traces radiate from the center of the frame to its four edges. Achip is carefully aligned over the center of the TAB frame, so that theleads or contacts of the chip are precisely mated with the correspondingmetal traces in the central portion of the TAB frame. In previousintegrated circuit devices, the chip is attached to the TAB frame with abonding agent. This connection of the chip contacts to the inner portionof the TAB frame is referred to as "inner-lead bonding" or "ILB"technology. The outermost sections of the conductive traces on the TABframe must also be carefully aligned with a similar set of correspondingtraces formed on a substrate. The substrate is attached to the TAB frameon the side that is opposite from the side that is adjacent to the chip.The connection of the outer portion of the TAB frame to a substrate likea printed circuit board pertains to "outer-lead bonding," often called"OLB" technology. The resulting assembly comprising the chip, the TABframe, and the substrate is essentially a space transformer that employsdiverging radial electrical pathways to afford ready access to theintegrated circuit.

Conventional manufacturing operations utilize soldering orthermocompression equipment to form metallurgical junctions between thechip contacts and the planar traces as well as between the TAB tracesand the substrate. Great quantities of heat and pressure must be appliedto the chip, TAB frame, and substrate during these mating processes.Excessive heat and pressure can seriously damage or even destroy thefragile circuitry within the chip. Highly accurate lasers capable ofsupplying radiant energy with great precision can be employed todiminish the chances for damage due to stray heat, but the basic problemof keeping the chip out of harm's way during a rapid automatedmanufacturing operation still remains unsolved. The heat expended duringconventional procedures also tends to cause the metal and plasticcomponents within the assembly to expand. The resulting deformations caneasily ruin the registration of the contacts and the traces. Even ifengineers and technicians can manage to coax acceptable yield rates fromthese methods of fabrication, the finished product is virtuallyimpossible to repair or to service. As chips become more complicated andmore expensive, it becomes less and less desirable to throw away entirechip, TAB frame, and substrate assemblies when a breakdown isencountered.

The problem of providing reliable chip-to-TAB frame and TABframe-to-substrate connection systems that avoid the detrimental sideeffects that inherently accompany soldering and thermocompressionbonding has presented a major challenge to designers in the automatedelectronic packaging field. The development of a relatively low cost,rugged, and versatile system that does not suffer from the impedimentsof misalignment and the inability to repair and replace defective chipswould constitute a major technological advance in the electronicsbusiness. The enhanced performance that could be achieved using such aninnovative device would satisfy a long felt need within the industry andwould enable electronic equipment manufacturers to save substantialexpenditures of time and money.

SUMMARY OF THE INVENTION

The Demountable Tape-Automated Bonding System disclosed and claimed inthis patent application solves the problems encountered by conventionalchip-to-TAB frame and TAB frame-to-substrate interface systems. Thedeleterious consequences of using solder and bonding agents for both OLBand ILB connections can be overcome by a modular, pitch-independent,self-aligned design that allows for virtually unlimited disassembly andreassembly. A chip may be either bonded or mounted in place atop adielectric TAB frame bearing a highly-conductive, corrosion-resistantcoating of conductive traces. Closely-spaced conductors protruding fromthe chip are held against the traces on the TAB frame. The fine traceson the TAB frame are maintained in precise registration withcorresponding traces on the substrate, that are, in turn, connected toexternal devices. The outer leads of the TAB frame are pressed againstthe substrate traces by compressive forces supplied by a cap and thesubstrate. Alternative embodiments of the present invention includeoptional heat sinks and heat spreaders. A bottom frame may be affixed tothe substrate to supply additional rigidity to the assembly. One of theembodiments of the invention includes an elastomer inserted between thecap and the substrate which provides the required contact forces betweenthe conductors on the TAB frame and those on the substrate.

The Applicants' invention is a highly serviceable chip interface devicethat is immune from the serious problems caused by soldering andthermocompression bonding. This innovative method and apparatus providean effective, efficient, and powerful tool that will enablemanufacturers of electronic equipment to create high quality productsthat will benefit a wide variety of consumers of the electronic andcomputer systems.

An appreciation of other aims and objectives of the present inventionand a more complete and comprehensive understanding of this inventionmay be achieved by studying the following description of a preferredembodiment and by referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a cross-sectional view of the basic embodiment of theDemountable Tape-Automated Bonding System which includes a heat sink.FIG. 1(b) is an enlarged view of a portion of FIG. 1(a) that reveals thedetail of the connection between a chip and the conductor layer of theTAB frame.

FIG. 2 is a schematic, exploded view of an isometric illustration of theembodiment depicted in FIG. 1.

FIG. 3 is a partial cross-sectional diagram that portrays a secondembodiment of the present invention which includes a cap and a bottomhaving orthogonal end portions.

FIG. 4 is a cross-sectional view of a third alternative embodiment ofthe Demountable Tape-Automated Bonding System which utilizes adouble-conductor TAB with a lower conductor layer coupled to a signalline.

FIG. 5 is a cross-sectional view of a fourth alternative embodiment ofthe present invention showing a double-conductor TAB with a via throughthe dielectric layer that is coupled to a ground line.

FIG. 6 is a cross-sectional view of fifth alternative embodiment of thepresent invention depicting a double-conductor TAB with a lowerconductor layer coupled to a ground line.

FIG. 7 is a cross-sectional view of sixth alternative embodiment of theDemountable Tape-Automated Bonding System which reveals adouble-conductor TAB with a via through the dielectric layer coupled toa signal line.

FIG. 8 is an exploded isometric view of another embodiment of thepresent invention that includes an elastomer above the TAB traces.

FIG. 9 is an exploded isometric view of yet another embodiment of theinvention which utilizes a clamp, an elastomer, and a heat spreaderarranged above the chip.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1(a) is a cross-sectional view of the basic embodiment of theDemountable Tape-Automated Bonding System 10. A chip 12 havingclosely-spaced contacts 13 protruding from it resides atop atape-automated bonding or "TAB" frame 14. As shown more clearly in theenlargement provided by FIG. 1(b), the TAB frame includes a dielectriclayer 14a and a conductive layer 14b which engages the contacts 13. Inthe preferred embodiment, the TAB frame 14 is constructed from two milthick polyimide film. The arrangement of the chip 12 and TAB frame 14 ismaintained by a standard permanent bonding method, such asthermocompression welding, or by the compression supplied by astainless-steel cap 18 and a metal substrate 16. In this embodiment,four alignment pins 29 maintain accurate positioning of the TAB frame 14to the substrate 16. Both the cap 18 and the substrate 16 may befashioned from metal, plastic, or a combination of both. In somecircumstances, the substrate 16 may be a printed circuit board. A bottomplate 20 is affixed beneath the substrate 16 to add strength andrigidity to the entire assembly 10. An optional heat sink 22 is shownextending above the cap 18.

FIG. 2 is a schematic, exploded view of an isometric illustration of theembodiment depicted in FIG. 1. Conductive traces 15 formed on the TABframe 14 and corresponding traces 17 deposited around the fourrectilinear edges of the substrate 16 are best seen in the seconddrawing figure. One of the TAB frames 14 constructed by the inventorsincludes a conductive pattern 15 of 520 eight mil pitch traces. Thetraces 15 are 0.5 ounce/square foot copper coated with a layer of goldover a nickel underlayer. The screws 28 which fasten the cap 18, TABframe 14, substrate 16, and bottom plate 20 are shown aboveprecision-drilled holes 26. Recesses 24 formed at the four corners ofthe substrate 16 accommodate the heads of screws 28.

FIG. 3 is a partial cross-sectional diagram that portrays an alternativeembodiment 30 of the present invention which includes a cap 18 and abottom 20 having orthogonal end portions.

FIG. 4 is a cross-sectional view of another alternative embodiment 34 ofthe Demountable Tape-Automated Bonding System 10 which utilizes adouble-conductor TAB frame 36, which includes an upper conductor layer36a, a lower conductor layer 36c, and a dielectric layer 36b sandwichedbetween them. The lower conductor layer 36c is held against an uppersignal line 38 that is connected to a vertical via or plated-throughhole 40, that is, in turn, connected to a lower signal line 42 embeddedin the substrate 16. Upper conductor layer 36a is held against an upperground line 44 by cap 18.

FIG. 5 is a cross-sectional view of a fourth alternative embodiment ofthe present invention showing a slightly different double-conductor TABframe 36. Lower conductor 36c impinges upon upper signal line 38 formedon the surface of substrate 16. Via 40 connects upper signal line 38 tolower signal line 42. Upper conductor 36a is coupled to ground line 44through a via 46.

FIG. 6 is a cross-sectional view of fifth alternative embodiment of thepresent invention depicting a double-conductor TAB frame 14 that alsoutilizes a pair of dielectric layers. Cap 18 forces upper dielectriclayer 36b to urge conductive layer 36a against upper signal line 38.Lower conductive layer 36c, which is isolated from the upper conductorlayer 36a by lower dielectric layer 36d, is coupled to upper signal line44, via 40, and lower signal line 43.

FIG. 7 is a cross-sectional view of sixth alternative embodiment of theDemountable Tape-Automated Bonding System 10 which reveals yet anotherdouble-conductor, double-dielectric TAB frame 14. A via 46 connectsupper conductor layer 36a to upper signal line 38, while lower conductorlayer 36c is impressed upon ground line 44.

Each of the embodiments illustrated by FIGS. 4 through 7 is a variationthat enables the designer to select different configurations thatprovide a wide spectrum of controlled impedance environments.

FIG. 8 is an exploded isometric view of still another embodiment of thepresent invention that includes an elastomer 49 consisting of thirty milthick Viton placed above the chip 12. Any elastomer material whichsoftens the compressive effects of the cap 18 and substrate 16 andallows for just the right amount of squeezing force may be employed whenpracticing the present invention.

FIG. 9 is an exploded isometric view of yet another embodiment of theinvention which utilizes a clamp 18 having a central hole 50, anelastomer 49, and a heat spreader 51 that fits within hole 50 arrangedabove the chip 12. Screws 52 fasten the clamp or lid 18 to heat spreader51, while screws 26 maintain the clamp 18, TAB frame 14, and printedcircuit board 20 in an accurate mechanical connection. PC board 20 isshown below the TAB frame 14, where traces 15 are mated to similarelectrical pathways 17 on the PC board 20. The TAB frame 14 is installedon the substrate 16 after careful alignment is insured.

The Demountable Tape-Automated Bonding System offers a rugged andreliable chip interface that may be used to great advantage in a greatnumber electronic products. This invention constitutes a major stepforward in the continually evolving field of electronic packaging andboard design.

Although the present invention has been described in detail withreference to a particular preferred embodiment, persons possessingordinary skill in the art to which this invention pertains willappreciate that various modifications and enhancements may be madewithout departing from the spirit and scope of the claims that follow.

LIST OF REFERENCE NUMERALS

10 Demountable Tape-Automated Bonding System

12 Chip

13 Contact protruding downward from chip

14 TAB (Tape-Automated Bonding Frame)

14a Dielectric portion of TAB

14b Conductive portion of TAB

15 Conductive traces on TAB

16 Substrate

17 Conductive traces on substrate

18 Cap

20 Bottom

21 Exploded view of Demountable Tape-Automated Bonding System

22 Heat sink

24 Recess at corner of substrate to accommodate screwhead

26 Screw hole

28 Screw

29 Guide pins

30 Cross-sectional view of Alternative embodiment of DemountableTape-Automated Bonding system

32 Conductive pad on surface of substrate

34 Cross-sectional view of Alternative embodiment of DemountableTape-Automated Bonding System showing Double-Conductor TAB with lowerconductor layer coupled to Signal Line

36 Double-Conductor TAB

36a Upper conductor layer

36b Dielectric layer

36c Lower conductor layer

36d Dielectric layer

38 Upper signal line

40 Via

42 Lower signal line

44 Ground line

45 Cross-sectional view of Alternative embodiment of DemountableTape-Automated Bonding System showing Double-Conductor TAB with viathrough dielectric layer coupled to Ground Line

46 Via through dielectric layer of TAB

47 Cross-sectional view of Alternative embodiment of DemountableTape-Automated Bonding System showing Double-Conductor TAB with lowerconductor layer coupled to Ground Line

48 Cross-sectional view of Alternative embodiment of DemountableTape-Automated Bonding System showing Double-Conductor TAB with viathrough dielectric layer coupled to Signal Line

49 Elastomer

50 Hole for heat spreader

51 Heat spreader

52 Screws attaching clamp to heat spreader

54 Screw holes through heat spreader for attachment to clamp

56 Conductor pad array

What is claimed is:
 1. A method of connecting a chip to a plurality ofconductive traces comprising the steps of:providing a substrate bearinga first pattern of conductive traces; aligning a tape automated bondingframe having a second pattern of generally planar conductive traces withsaid substrate, said conductive traces having inner lead ends and outerlead ends, said aligning including registering said outer lead ends withsaid second pattern of conductive traces; securing said tape automatedbonding frame to said substrate, including electrically coupling saidouter lead ends to said second pattern of conductive traces; placing anintegrated circuit chip having a plurality of closely-spaced contactsatop said tape automated bonding frame so that contacts are joined tosaid inner lead ends; and fastening a cap over said chip to said tapeautomated bonding frame such that said contacts of said chip and saidinner lead ends are compressed together by forces applied at oppositesides thereof, thereby creating solderless electrical connectionsbetween said contacts of said chip and said inner lead ends.
 2. A methodas claimed in claim 1, further comprising:affixing a heat sink to saidcap that extends above said cap.
 3. A method as claimed in claim 1,further comprising:placing an elastomer between said cap and said tapeautomated bonding frame.
 4. The method of claim 1 further comprisingforming said tape automated bonding frame having said second pattern ofconductive leads such that said conductive leads are planar metallicmembers disposed on a planar dielectric material.
 5. A method ofconnecting a plurality of contacts to a plurality of conductive tracescomprising the steps of:providing a rigid substrate having a firstpattern of conductive traces; aligning a tape automated bonding framehaving pliable conductive leads with said first pattern of conductivetraces such that said conductive leads have first ends in contact withsaid conductive traces; and coupling a cap to said rigid substrate on aside of said tape automated bonding frame opposite to said rigidsubstrate, said coupling including positioning said cap such that saidfirst lead ends are compressed against said conductive traces of saidrigid substrate, thereby achieving a solderless electrical connectionbetween said first lead ends and said conductive traces.
 6. The methodof claim 5 further comprising placing an elastomeric member between saidcap and said tape automated bonding frame.
 7. The method of claim 5further comprising bonding second lead ends of said tape automatedbonding frame to an integrated circuit chip.